Influence of overriding plate velocity changes on slab dip and deformation: insights from laboratory models

International audienceOver geological times, plate reorganization associated with mantle convection led to changes in absolute platevelocities, which may in turn have impacted the geometry of the subducting plate as well as the overriding plateregime of deformation. Indeed, previous studies have shown a very good correlation between the absolute motionof the overriding plate on one hand and slab dip and overriding plate deformation on the other hand: extensionand steep slab are associated with an overriding plate moving away from the trench while shortening and shallowslab occur if the upper plate goes the other way. However, these correlations are established when subduction hasreached a steady-state regime and for a constant motion of the overriding plate over the subducting plate, whichmay not always be the case on Earth. The response of the subduction system to changes in absolute overridingplate velocity still remain an open question.In this study, we conducted a set of 3-D mantle-scale laboratory models of subduction in which we incrementallychanged the velocity of the overriding plate to reproduce changes of velocities that may arise from variations offar-field boundary conditions in Nature. We first show that strain rates in the overriding plate are correlated withoverriding plate absolute velocity but also that the regime of deformation adjusts rapidly to changes of velocity.This may explain for instance why despite the subduction has been continuous beneath South America since atleast the middle Jurassic, shortening along its active margin is only recorded episodically, the main phases ofAndean orogeny roughly corresponding to periods of South American plate westward acceleration. We also showthat slab dip adjusts to changes of overriding plate velocity but it requires several Myr before it stabilizes. It mayexplain why the correlation between absolute overriding plate motion and slab dip from the analysis of present-daysubduction zones is only moderate, part of the subduction zones possibly not being at steady-state at present-day